Mechanical primary vacuum pumps include a motor-driven displacement member such as a to-and-fro moving piston, one or more rotary pistons, a rotary flap or a sliding flap. A spring-loaded non-return flap valve is often provided in such a pump, at least in the outlet channel, in order to maintain the achieved vacuum pressure after the shut-off of the pump in a tank which has been evacuated. Mechanical primary pumps are often combined with pumps of other types, such as diffusion or molecular pumps, in order to reach a higher vacuum, the mechanical pump then acting as primary pump and operating against ambient pressure. In some cases, a jet pump such as a water ring pump has been inserted into the outlet channel of the mechanical vacuum pump, which signifies that the mechanical pump no more operates against ambient pressure, but against a significantly lower pressure, which allows to increase the quality of the final vacuum to be achieved by the mechanical pump.
French patent application No. 24 26 171 discloses a mechanical vacuum pump which is combined with a jet pump. The latter, however, is only operative when the mechanical vacuum pump is not operative. The German patent document DE-12 70 215 B and the German patent document DE-16 78 604 A disclose vacuum pumping devices including a mechanical primary pump and a jet pump connected in series, the latter being located between the tank to be evacuated and the mechanical primary pump and operating as secondary pump.